help second display stutter when set as primary

[UpsideDownX]

hi guys this is my first time posting here so if there's anything wrong please tell me and i'll adjust, and also english is not my native language so if you don't understand anything that I've typed feel free to ask.

 

So, i just got a new monitor, Model: Dell P2319h, everything seems fine, new in box directly from dell itself, the problem only appears when i connect it to my laptop (a dell 2in1 from 2016) and set the external display to be my primary display, the screen is set to 60hz and i can feel that the cursor is actually running smoothly, but when i run any video on youtube at 60hz the video stutters constantly, so i ran the ufo test and fair enough it reads 33~35hz and stutters like crazy (to confirm if it was a monitor problem i connected my laptop to a tv and the problem showed up there too)
now, the weird thing is, this only happens if the Dell monitor is set in windows as the primary display(in extended mode or if i set it to be the only display for the system) but if i set to mirror or extended but the primary display been the built-in laptop display the problem disappears, videos on yt run smoothly and UFO's test detects 60hz.
do you guys know what is happening here if its a port problem or a software problem, and how if possible i can fix it?

thanks in advance!